The present invention relates to a semiconductor process, specifically, to a novel termination structure for power MOS devices so as to prevent leakage current.
Doubled diffused metal-oxide-semiconductor field effect transistor (DMOSFET), insulated gate bipolar transistor (IGBT), and Schottky diode are important power devices and use extensively as output rectifiers in switching-mode power supplies and in other high-speed power switching applications. For example, the applications include motor drives, switching of communication device, industry automation and electronic automation. The power devices are usually required carrying large forward current, high reverse-biased blocking voltage, such as above 30 volt, and minimizing the reverse-biased leakage current. There are several reports that trench DMOS, trench IGBT and trench Schottky diode are superior to those of with planar structure.
Among the power device, the IGBT is the most commercially devices. Typical trench IGBT power device is illustrated in FIG. 1, a schematic cross-sectional view. In the figure, a substrate is shown which comprises a p type doping semiconductor substrate 10 having an collector electrode 11 on an opposite face thereof and a n+ buffer region 13, an n-drift region 14 sequentially deposed thereon. In addition, n-drift regions 14 having a p-well region 15 deposed thereon. A trench gate structure has a gate oxide layer 16 therein and a bottom deeply formed into n-drift region 14 of the substrate and, an insulting layer 17 is formed to isolate the conductive layer 18 from the emitter electrode 19. The emitter electrode 19 contacts to the n+ regions and p+ regions. Both regions are in the pitch of trench gate structure and on the p-diffused well regions 15. The p-type substrate 10, the n-type buffer 13 and drift regions 14, and p diffused well region 15 collectively form the collector, base and emitter of a vertical p-n-p bipolar transistor.
For power transistors are concerned, the described power device for carrying large current are susceptible to premature breakdown because the p-diffused well region do not form semi-infinite parallel -plane p-n-junction with the drift region. It is required to consider the edge effects to obtain realistic design. Hence a termination structure design in the periphery of the active region is usually at an end of a die so as to prevent voltage breakdown phenomena from premature.
FIG.2A shows a planar diffused termination. In the case, the electric field (indicated by arrows) crowding are expected to occur at the edge of cylindrical junction 30. Hence, the breakdown of planar diffused junction would be occurred at edge rather in the parallel-plane portion. FIG. 2B shows a field ring 31 fabricated simultaneously with a main junction 32 of a power device. In the case, the electric field (indicated by arrows) and depletion field contour 33 are shown. Comparison of electric field crowding of FIG. 2B with 2A, it cab be seen that the electric field crowding responsible for the premature breakdown voltage is substantial reduced by the presence of field ring region 31.
FIG. 3 shows another sort of field plate 42 termination structure. In the figure, a main junction 40 and a field plate 42 formed on an insulating region 41. A depletion layer 45 indicated by a dotted line may be generated in the substrate 35. Similarly to field ring structure, the presence of the field plate 42 may be able to support a breakdown voltage in a range of about 60% of the ideal breakdown voltage of a parallel-plane junction. In the case, if the insulating layer 41 is thick, breakdown will typically occur in region xe2x80x9cAxe2x80x9d, however if the insulating layer 41 is thin then breakdown will typically occur in region xe2x80x9cBxe2x80x9d.
Since a single field plate and a field ring can reduces depletion layer curvature and electric field crowding. It can be expected that several field plate and field rings working in conjunction with each other may raise the breakdown voltage even closer to the parallel plane case. FIG. 4 shows such forging concept application, a termination design with multiple field plates 50 and field rings 60A, 60B, and 60C. In the case, the spacing s1, s2, and s3 between individual floating filed plate 50 and the width w1, w2, w3 of each field rings 60A, 60B and 60C are varied. Both spacing and width are decreased with increasing distance with main junction 55. If the surface space chares are precisely known, the spacing and width can be aptly selected, the depletion boundary 65 is as illustrated in the figure.
Recently, to prevent low breakdown voltage, another approaching is proposed by Seok in the U.S. Pat. No. 5,731,627 issued on Mar. 24, 1998. In the reference, see FIG. 5, the termination structure comprises overlapping floating field plates. They include a primary field plate 76a and a plurality of floating field plates 76b, 76c, 76d which are formed on an electrically insulating region 66 and capacitively coupled together in series between an active region of a power semiconductor device and a floating field ring 72b. 
As forgoing several conventional termination structures, though solve the electric field crowding problems at the edge so that the premature breakdown voltage are substantial improved. However, they usually demands long termination length and hence spend much planar area. An object of the present invention thus proposes a novel termination structure. With the new termination structure the bending region of the depletion region are away from the active region without long termination length, and thus save much planar area.
The present invention discloses a termination structure method for IGBT power devices and a method making the same. A power device termination structure is disclosed. The structure comprises a primary field plate electrically connect to a main junction, a secondary field plate electrically connect to a field ring which are apart from the main junction, and a floating field plate formed in between the primary field plate and secondary field plate. The primary field plate and secondary field plate are formed on an insulating layer, and the floating field plate is buried in the insulating layer. The endings of floating field plate are in alignment with the ends of the extension portion of the primary field plate and the secondary field plate. The primary field plate, the secondary field plate and the floating conductive plate, are capacitively coupled each other so that the electrical field crowding problem is lesser. Since the termination structure requires only a floating field plate in conjunction with a primary field plate and a secondary field plate and hence reduces significantly the termination area.